Generally, a Diesel engine is mounted on construction machines to serve as a prime mover for driving hydraulic pumps.
In this regard, it has been the general practice for conventional construction machines of this to provide a control lever in an operator's cabin and to link the control lever with a governor mechanism of the engine through a controlling cable, link rod and so forth for control the rotational speed of the engine. However, the mechanical linkage of the control lever with the governor mechanism through the control cable and link rod has a drawback that it requires large operating forces.
With a view to eliminating such a drawback, there has been proposed an electric remote control system for governor mechanism, including an electric motor provided in the vicinity of an engine for governor adjustment, a rotational angle sensor adapted to detect the rotational angle of the governor mechanism indicative of the rotational speed of the engine, and a command means in the form of operating switches or the like provided in the operator's cabin in association with a controller such as microcomputer. The controller is adapted to control the electric motor through feedback control in such a manner so as to make the difference between a signal value specified by the command means and a signal value detected by the rotational angle sensor zero, thereby turning the governor lever of the governor mechanism to a position corresponding to the specified value.
In this connection, FIGS. 11 to 13 show, by way of example a construction machine employing a prior art prime mover rotational speed control system with a governor mechanism of the type mentioned above.
In FIGS. 11-13, a Diesel engine 1 is mounted on a construction machine as a prime mover with governor 2 being provided on the engine 1. The governor 2 includes an elongated governor lever 3 and stoppers 4 and 5 for limiting the rotational range of the governor lever 3 by abutting engagement therewith. The governor 2 functions to adjust the rotational speed of the engine 1 according to the rotational angle of the governor lever 3 in an accelerating direction H or decelerating direction L, and, as shown in FIG. 12, to hold the engine at the lowest speed N.sub.L (idling speed) when the governor lever 3 abuts against the stopper 4 where the value of lever rotation is 0%, while holding the engine at the maximum speed N.sub.H (full speed) when the governor lever 3 abuts against the stopper 5 where the value of lever rotation is 100%.
A reversible stepping motor is mounted in the vicinity of the engine 1. A lever 6A is mounted on the output shaft of the stepping motor 6 and is connected to the governor lever 3 through a link 7. The stepping motor 6 is rotatable in a forward direction F or in a reverse direction R in dependence upon a control pulse signal from a controller 10, which will be described hereinbelow to thereby rotate the governor lever 3 in the accelerating direction H or in the decelerating direction L through the link 7. Even when the rotation of the governor level is stopped by a stop signal received from the controller 10, the governor lever 3 is retained in the current angular position to operate the engine 1 at the current rotational speed.
A potentiometer 8 is provided in the vicinity of the engine 1 to serve as a rotational angle sensor. A lever 8A is mounted on a rotational shaft of the potentiometer 8 and is connected to the link 7. The potentiometer 8 is preadjusted such that its detection range (output range) is held in a predetermined relationship with the rotational range of the governor lever 3 as indicated by solid line in FIG. 12. The potentiometer 8 is adapted to detect the rotational angle of the governor lever 3 through the lever 8A and link 7 to produce an output signal indicative of the rotational speed of the engine 1 for supply to the controller 10.
An up-down switch is provided in the operator's cabin of the construction machine as a command means for specifying a target engine speed. The up-down switch 9 is a push-button type up-switch and down-switch (not shown). The up-down switch 9 is adapted to supply the controller 10 with a command signal, namely, an acceleration command signal or a deceleration command signal corresponding to the extent of the depressive operation on the up- or down-switch 9. According to the received command signal, the controller 10 sets up a target value M which corresponds to the target rotational speed of the engine 1 as will be described hereinbelow.
The controller 10 includes arithmetic operation circuit like CPU and a memory circuit such as ROM and RAM (not shown) as well as a memory area 10A in the memory circuit. For setting up a target value M which corresponds to the target rotational speed of the engine 1, the controller 10 is adapted to convert the command signal from the up-down switch 9 into a percentage target value M as shown in the graphical illustration of FIG. 13, which is stored in the memory area 10A, and to store the target value M thus obtained. Then, the controller 10 compares the target value M with a value N.sub.B of governor lever rotation, which is detected by the potentiometer 8 and corresponds to the rotational speed of the engine 1, to produce a control pulse signal to the stepping motor 6. Accordingly, the stepping motor 6 rotates the governor lever 3 in the accelerating direction H or decelerating direction L to control the rotational speed of the engine 1 to the target value.
With a prime mover rotational speed control system of the above-described prior art construction, the operator enters a desired engine speed through the up-down switch 9, whereupon the controller sets up a target value M of the engine speed according to the command signal from the up-down switch 9. Then, the controller 10 reads in the rotational angle of the governor lever 3 from the potentiometer 8 as a value corresponding to the current rotational speed of the engine 1, comparing the value with the target value M to produce a control pulse signal to be applied to the stepping motor 6 for rotation in the forward or reverse direction. As a result, the governor lever 3 is turned in the accelerating direction H or decelerating direction L to adjust the engine speed in accordance with the target value M.
As soon as the rotational speed of the engine 1 substantially reaches the target value M, the controller 10 produces a stop signal as a control pulse signal for the stepping motor 6, which then maintains the governor lever 3 at the current rotational angle to enable the engine 1 rotate at a speed corresponding to the target value M.
In this regard, the above-mentioned prior art is arranged to compare the target value M with a value N.sub.B of governor lever rotation, which is detected by the potentiometer 8 as an indicator of the rotational speed of the engine 1, and to adjust the rotation of the stepping motor 6 for control of the rotational speed of the engine 1. It follows that, in the entire rotational range between the minimum and maximum rotational speeds which are delimited by the stoppers 4 and 5, the governor lever 3 must be turned in a manner which corresponds to the detection range of the potentiometer 8 as indicated by solid line in FIG. 12.
However, in the above-described prior art, the positions of the stoppers 4 and 5 differ from engine to engine, so that it becomes necessary to preadjust the range between these members by changing the setting of the link ratio or through fine adjustment of the potentiometer 8 individually for each engine. These preadjustments are very troublesome and time consuming. Besides, there is a problem that the governor lever 3 and link 7 are susceptible to loosening of mechanical parts as a result of repeated operations over a long period of time, or a problem that temperature variations might cause variations in output characteristics of the potentiometer 8, resulting in a difference between the rotational range of the governor lever 3 and the detection range of the potentiometer 8, for example, as indicated by broken line in FIG. 12 to make correct control of the engine speed difficult. Furthermore, there are possibilities of noises creeping into the detection signal from the potentiometer to lower the accuracy and reliability of the engine speed control.